Simple hack to emulate bash/zsh's HISTCONTROL=ignorespace

[kvs]

A leading space on a command will skip saving in history.

I thought about making this configurable, or adding an option
to the history builtin, but it all just seemed too complex for
such a simple thing.

An alternative would've been a built-in command like 'nohist'
or somesuch, but that also seemed excessive.

[ridiculousfish]

Merged here:

To git@github.com:fish-shell/fish-shell.git
1afdbb8..4cce303 master -> master

I took the documentation commit (thanks for remembering to do that) but 09e5ebe had a few issues:

• should use .at(0) instead of c_str()[0]
• should compare against a wchar_t: L' '

So I implemented those. Thanks!